Isolated ureteral strips develop spontaneous phasic contractile activity which is enhanced by 5-hydroytryptamine (5-HT). The aim of this study was to identify the receptor subtype mediating these responses and to determine whether responses to 5-HT change with age. The frequency of contractions and the overall contractile activity (measured as the area under the curve, AUC) were recorded in strips of porcine distal ureter isolated from young (3 months) and old (2 years) pigs. Responses to 5-HT were examined in the absence and presence of selective 5-HT receptor subtype antagonists. Tissues from the younger animals elicited larger contractile responses to 5-HT (5885 ± 335g-1 s) than tissues from the older animals (2787 ± 317g-1 s, P < 0.001). The 5-HT2A receptor antagonist ketanserin (10-100nM) elicited rightward shifts of 5-HT concentration-response curves, antagonising AUC and frequency responses with high affinity in tissues from both age groups (pKD values 8.4-8.8). The slopes of the corresponding Schild plots were not significantly different from unity, suggesting a competitive antagonism at a single receptor, except for frequency responses to 5-HT in the older animals. Antagonists selective for other 5-HT receptor subtypes: methiothepin (non-selective), RS-10221 (5-HT2C), ondansetron (5-HT3), GR-113808 (5-HT4), SB699551 (5-HT5), SB399885 (5-HT6), SB269970 (5-HT7)) had no effect on 5-HT-induced responses. The results suggest that the 5-HT2A receptor subtype is the predominant receptor mediating 5-HT responses in ureteral tissues, being the sole mediator of responses in tissues from young animals, but with another receptor subtype also playing a minor role in the older animals.